Effects of binder component and curing regime on compressive strength, capillary water absorption, shrinkage and pore structure of geopolymer mortars

To address the slow hardening and poor performance of low-calcium fly ash (FA) geopolymers at ambient temperatures, this study investigated the effects of silica fume (SF), ordinary Portland cement (OPC) content, and curing regimes on the compressive strength, capillary water absorption, and drying shrinkage of fly ash-based geopolymer mortar (FA-GPM). Microstructural analysis was used to examine the evolution of the microstructure and hydration products. Additionally, Nuclear Magnetic Resonance (NMR) testing analyzed the effects on pore size distribution and porosity of the geopolymer mortar (GPM). The results showed that GPM with a 30 % replacement of SF and OPC achieved the highest 28-day compressive strength of 50.52 MPa, a 78.39 % increase compared to the specimen with 0 % replacement. Among the curing regimes, steam curing at 60°C resulted in the highest compressive strength. It was also confirmed that 12-hour carbonation curing benefits compressive strength development. However, excessively high-temperature steam curing and prolonged carbonation curing may damage the GPM matrix, adversely affecting performance. NMR results showed that adding OPC and SF reduced the number of pores in the mortar, gradually transforming harmful pores into harmless ones, resulting in a denser structure. Microstructural analysis revealed that adding OPC and SF promoted the formation of sodium-alumino-silicate-hydrate (N-A-S-H) and calcium-alumino-silicate-hydrate (C-(A)-S-H) gels, with C-(A)-S-H significantly impacting the strength and pore structure of FA-GPM. Notably, carbonation curing formed more strength-contributing gels, with the carbonates combining with the gels to form a binding phase that retains the original structure. The curing regime influences GPM performance by altering the quantity of gel products. This study highlights the benefits of adding SF and OPC to FA-GPM and identifies low-carbon, environmentally friendly curing regimes, providing new perspectives for research and large-scale applications in FA-based geopolymers. •The addition of OPC and SF significantly increased the compressive strength of the GPM.•In binder ratios, the mixture with the best strength also has the best shrinkage and capillary water absorption.•Reasonable carbonation curing can well improve the capillary water absorption, shrinkage and pore structure of GPM.•Microanalysis showed that the gels were mainly C-(A)-S-H and N-A-S-H. The C-(A)-S-H gel had a large effect on the propert